Carrier aggregation is a technique available for increasing the bandwidth available for communication by employing simultaneously more than one carrier for communication by a single communication device. The carriers may be in different radio frequency bands, in which case they occupy portions of spectrum that are spaced apart, or may occupy contiguous portions of spectrum in a single radio frequency band, or a combination of both possibilities, occupying contiguous portions of different radio frequency bands. A further possibility is that the carriers occupy a single radio frequency band, but not all of the carriers occupy portions of spectrum that are contiguous, there being one or more gaps in the spectrum occupied by the carriers. The present disclosure addresses, in particular, such carrier aggregation in which the carriers are non-contiguous within a single frequency band.
Carrier aggregation has been introduced in release 8 of the 3GPP High Speed Downlink Packet Access protocol, commonly referred to as HSDPA. In release 8 (rel-8) of HSDPA, only two adjacent carriers can be aggregated. Release 9 (rel-9) introduces the possibility to schedule two carriers in two different bands, that is, one carrier in one band and one carrier in an other band, for example band I and band VIII. In release 10 (rel-10) up to four carriers can be aggregated which can be located in the same band, with a maximum of three adjacent carriers being considered, or in two different bands. In LTE, carrier aggregation has been introduced in rel-10. Up to rel-10, only the aggregation of contiguous portions of the spectrum is possible, within one band. With release 11 (rel-11), non-contiguous carrier aggregation is possible.
FIG. 1 illustrates some configurations of carriers which may be employed for non-contiguous carrier aggregation in the High Speed Downlink Pack Access (HSDPA) protocol. Referring to FIG. 1, eight scenarios A to H are represented, with scenarios A to C being applicable in Band I of HSDPA and scenarios D to H being applicable in Band IV of HSDPA. Each carrier occupies 5 MHz of spectrum, and the gap between those carriers that may be used for carrier aggregation is, 5 MHz, or a multiple of 5 MHz. In practice, the gaps may be occupied by carriers belonging to a different communications network. For the purpose of this document, such a carrier that occupies the gap will be referred to as an interferer, interference signal or an unwanted signal, as it carries no information intended for a receiver using the non-contiguous carriers for communication. In scenario A, two carriers separated by a 5 MHz gap are aggregated, and this configuration is denoted C-5-C. In scenario B, three carriers are aggregated with a gap of 5 MHz between the first and second carriers, which are the two carriers of lowest frequency, and this configuration is denoted C-5-CC. In scenario C, four carriers are aggregated with a 10 MHz gap between the first and second carriers, and this configuration is denoted C-10-CCC. Scenario D has an identical configuration to scenario A. Scenario E has three carriers with a 10 MHz gap between the first and second carrier, this configuration being denoted C-10-CC. Scenarios F and G both have four carriers and a central gap of 15 MHz and 20 MHz, being denoted, respectively, CC-15-CC and CC-20-CC, with the gap being between the second and third carriers. Scenario G has three carriers with a 20 MHz gap between the second and third carriers, which are the two carriers of highest frequency, and is denoted CC-20-C. Some of the configurations in FIG. 1 span 20 MHz or less, and others span more than 20 MHz.
Mobile communication networks which conform with the 3GPP LTE protocol can employ carrier spacings of 1.4, 3, 5, 10, 15 and 20 MHz, depending on the spectrum conditions and availability, and where such a network implements carrier aggregation, the gap between non-contiguous carriers can be an integer multiple of one of these spacings.
Also, user equipments (UEs) may implement both HSDPA and LTE technologies and hence reuse of the components between the two different radio access technologies (RATs) is highly desirable.
The gap between non-contiguous carriers may be occupied by a carrier transmitted by a different communication network, which may be regarded as an unwanted carrier with respect to a receiver receiving the non-contiguous carriers. Such an unwanted carrier may therefore also be regarded as an interference signal. Indeed, it is very likely that an interferer will be present in the gap. The interferer can be from a different operator that may deploy the same radio access technology (RAT) or another RAT in the gap. For example, scenario B in FIG. 1 is a typical scenario for Telecom Italia, and the gap is occupied by Vodafone. Moreover the assumption of similar power received in the wanted carrier and in the gap does hold true only when geographical co-location of the two operators is possible. This is not the case in practice for most of the operators deploying their network in the same area. So, whereas systems employing carrier aggregation may control the relative levels of the contiguous carriers, with non-contiguous carriers, the level of an interference signal in a gap may not be controllable by the system and may be relatively high.
In a wireless communication network employing carrier aggregation, the selection of carriers to be aggregated may be based on selection criterion that takes account of the signal quality of candidate carriers. For example, a mobile terminal may measure the quality of candidate carriers and report the result of the measurement to a network node, which can then employ the result in selecting the carriers to be aggregated.
In order to reduce the affect of an interference signal in a gap, a receiver may employ dual local oscillators and dual signal paths, with one local oscillator, mixer and filter being used for down-converting carriers which lie on one side of a gap, and a second local oscillator, mixer and filter being used for down-converting carriers which lie on the other side of the gap. Such a receiver can be complex, large, and have relatively high power consumption, in comparison with a receiver arranged for receiving only contiguous carriers.
In order to avoid a high complexity, increased size and higher power consumption of using dual local oscillators and dual signal paths, a receiver with a single local oscillator and single signal path may be tuned to each candidate carrier sequentially to measure signal quality. However, such a scheme can be slow, particularly where many candidate carriers are measured, or where the measurements takes place at intervals during time gaps in ongoing communication.
Consider an example for a configuration C1 x C2 C3 x x x x C4, where the UE camps on C4. For this example we make the assumption of a one LO UE. A signalling method, is summarised as follows:                1) A network node (NodeB or eNB) requests inter-frequency measurements on carriers C1, C2 and C3        2) The UE uses one measurement gap, that is, a time gap, during which it changes the position of the LO and does measurement on C1        3) The UE reports the received signal strength indicator (RSSI) and energy-per-chip to noise-plus-interference ratio (Ec/No) for C1        4) The UE uses one measurement gap during which it changes the position of the LO and does measurement on C2        5) The UE reports the RSSI and Ec/No for C2        6) The UE uses one measurement gap during which it changes the position of the LO and does measurement on C3        7) The UE reports the RSSI and Ec/No for C3        
Note that each measurement is done on a 5 MHz bandwidth as it was done so far before HSDPA rel-8. With this method we need N gaps to measure each of the N carriers once.
We consider measurement methods in more detail using the example configuration C1 x C2 C3 x x x x C4 where the operator has the carriers C1, C2, C3, C4 and the UE is camped on C4. The term, ‘x’ denotes carriers belonging to other operators. In High Speed Packet Access (HSPA) each carrier is of 5 MHz. In LTE, the carriers can have the same or different bandwidths, for example any combination of 1.4, 3, 5, 10, 15 and 20 MHz carriers.
The signalling method used in the legacy for performing measurements on carriers on non-contiguous carrier aggregation scenario comprises:                The radio network node, for example a Radio Network Controller (RNC) in HSPA or eNodeB in LTE, requests the UE to make inter-frequency measurements on carriers C1, C2 and C3. The requirements are currently defined for two inter-frequency carriers in High Speed Packet Access (HSPA) and three inter-frequency carriers in LTE.        The UE reports the measurements for each carrier. For example, for a Universal Telecommunication System Terrestrial Radio Access (UTRA) carrier the UE reports measurements of RSSI, Common Pilot Channel (CPICH) or Received Signal Code Power (RSCP); for HSPA, the UE reports measurements of CPICH or Ec/No; for LTE, the UE reports Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ).        In HSPA the UE uses one measurement gap pattern, also known as compressed mode pattern, for each carrier during which it changes the position of the LO and does a measurement on that carrier. In HSPA the UE reports the measurements on each inter-frequency carrier, for example on C1, C2 and C3, when the measurements are done.        In LTE the UE uses one measurement gap pattern for measuring on all inter-frequency carriers. During each gap, which is 6 ms, or effectively 5 ms due to frequency switching, typically a UE measures one carrier at a time. This means the UE adjusts the position of the LO to be centred on a carrier, for example C1, and does measurement on that carrier. In LTE the UE reports the measurements on each inter-frequency carrier, for example on C1, C2 and C3, when the measurements are done.        
The measurements may be done by the UE on the serving as well as on neighbour cells over some known reference symbols or pilot sequences. Some measurements may also require the UE to measure the signals transmitted by the UE in the uplink. In a multi-carrier or carrier aggregation (CA) scenario, the UE may perform the measurements on the cells on the primary component carrier (PCC) as well as on the cells on one or more secondary component carriers (SCCs).
The measurements are done for various purposes. Some example measurement purposes are: mobility, positioning, self organising network (SON), minimisation of drive tests (MDT), operation and maintenance (O&M), network planning and optimisation. The measurements may also comprise cell identification, for example Physical Cell Identity (PCI) acquisition of the target cell, Cell Global Identity (CGI) or Evolved Cell Global Identity (ECGI) acquisition of the target cell, or system information acquisition of the target cell. The target cell can be of LTE or any inter-RAT cell. Examples of intra-frequency, inter-frequency and CA mobility measurements in LTE are RSRP and RSRQ. Examples of intra-frequency, inter-frequency and multi-carrier HSPA mobility measurements are: CPICH, RSCP, CPICH Ec/No and UTRA carrier RSSI. Examples of well known positioning UE measurements in LTE are Reference Signal Time Difference (RSTD) measurement and UE transmitter-receiver time difference measurement.
The measurements described above can be used to enable UE mobility. These measurements are also applicable for a UE camped on, or served by, a mobile relay.
There are two kinds of UE mobility states:
Low activity state mobility, for example cell selection and cell reselection
Connected state mobility, for example handover, cell change order, RRC connection release with re-direction, primary cell change, primary carrier change, and RRC connection re-establishment.
In LTE there is only one low activity mobility state, called idle state. In HSPA there are the following low activity states: Idle state; UTRA Registration Area Paging Channel (URA_PCH) state; Cell Paging Channel (CELL_PCH) state; and Cell Forward Access Channel (CELL_FACH) state. Nevertheless, in any low activity state the UE autonomously performs cell reselection without any direct intervention of the network. To some extent the UE behaviour in a low activity mobility state scenario could still be controlled by a number of broadcast system parameters and performance specification. In HSPA, the connected state is also known as the Cell Dedicated Channel (CELL_DCH) state since at least a dedicated channel (DCH) is in operation for at least the maintenance of the radio link quality.
Handover, on the other hand, is fully controlled by the network through explicit UE specific commands and by performance specification. Similarly the Radio Resource Control (RRC) re-direction upon connection release mechanism is used by the network to re-direct the UE to change to another cell, which may belong to the RAT of the serving cell or to another RAT. In this case the UE upon receiving the ‘RRC re-direction upon connection release’ command typically goes into an idle state, searches for the indicated cell or RAT, and accesses the new cell or RAT.
In both the low activity state and the connected state the mobility decisions are mainly based on the same kind of downlink neighbour cell measurements, which are described above.
Both Wideband Code Division Multiple Access (WCDMA) and Evolved UTRA Network (E-UTRAN) are frequency reuse-1 systems. This means the geographically closest or physical adjacent neighbour cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories:
Intra-frequency mobility (low activity and connected states)
Inter-frequency mobility (low activity and connected states)
Inter-RAT mobility (low activity and connected states)
In intra-frequency mobility, the UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay as mobility measurements can be carried out in parallel with channel reception. In addition, an operator would have at least one carrier at its disposal that it would like to be efficiently utilised.
In inter-frequency mobility, the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario.
In inter-RAT mobility, the UE moves between cells that belong to different access technologies such as between WCDMA and Global System for Mobile Communication (GSM) or vice versa, or between WCDMA and LTE or vice versa.
The most straightforward approach for receiving non-contiguous carriers is to use two separate receivers. However, there would be a cost benefit in facilitating a single-receiver UE implementation, for example, enabling re-use of LTE components and avoiding activating a second chain for configurations that could be supported with a single receiver. One possibility may be for operator A and operator B to perform some kind of joint coverage planning. Hence there may be prior knowledge that the situation of a relatively high power interferer in a gap between two non-contiguous carriers cannot occur. To allow this, it would seem necessary to specify the maximum power difference between wanted and unwanted carriers that can be tolerated by a single-receiver UE. In practice, it is only likely that this option can be considered if operator A and operator B cell sites are completely shared, so it does not cover all likely cases. Another approach would be to use network radio resource management (RRM) strategies to attempt to detect and avoid such a scenario. For example, the UE could be configured to two-carrier (2C) or single-carrier (1C) operation in locations where the operator B signal was too strong to allow non-contiguous four-carrier HSDPA (NC-4C-HSDPA) operation.
It could be beneficial for the UE to report that there is an imbalance condition RSCP, may not be suitable for this purpose, partly because operator A cannot provide to UEs, or know, a neighbour cell list for the operators on operator B spectrum, and partly because carrier RSSI rather than RSCP would be the relevant metric, especially as operator A is unaware of the loading of cells on operator B's spectrum. The carrier RSSI is measured over the entire carrier and not on a specific cell. Hence the UE does not require knowledge of the Physical Cell Identifier (PCI) or scrambling code used in the cells on the carrier.
Although inter-frequency detected set measurements of operator B's carrier could be considered, that is, measurements related to a carrier belonging to the detected or detectable set of carriers of operator B, the reporting of only one cell and the fact that load is unknown would be a significant limitation. A Channel Quality Indicator (CQI) of the serving cell may also give an indication of the presence of an interferer. However, the CQI is a short term metric and would require further averaging, for example in eNB, that is, eNodeB, to reflect the long term situation. This may delay the decision of the eNB to revert to an easier configuration, such as two carriers or a single carrier, which may cause some throughput degradation and some losses. Additionally, lub interface changes may be needed to provide information to the RNC to trigger mobility procedures based on CQI. The lub interface connects a Radio Network Controller (RNC) to a Node B base station.
Periodic inter-frequency RSSI reporting may be configured using existing measurement configuration mechanisms. Some possible issues are the number of carriers that can be monitored by the UE at the same time and the use of a maximum number of compressed mode gaps for the measurements. More significantly, the reporting rate in case of periodic reporting may need to be quite high to ensure a fast enough response to changing interference conditions to avoid call drop. The high reporting rate causes a high overhead in RRC signalling.